CA1189311A - Liquid crystalline chiral compound, compositions and guest-host display including same - Google Patents

Abstract

ABSTRACT

A novel nematic liquid crystalline composition is provided. A nove1 liquid crystalline chiral compound, (+)5-(2-methyl) butyl-2(4-cyanopheny 1)-1, 3-dioxane, is also provided. When admixed together, these novel com-ponents form a phase-change cholesteric liquid crystalline composition which is highly useful as the host medium in a guest-host electrooptical display as d result of its low electrical threshold and optical saturation voltages, steep saturation curve, relatively low optical birefringence, low viscosity and broad liquid crystalline mesomorphic range.

The host medium in conjunction with a suitable pleochroic dye provides an improved guest-host composition which exhibits a contrast of greater than 3:1 in certain displays, especially those having liquid crystal align-ment layer means providing a moderate tilt angle of 10°-40° at the sub-strate/guest-host composition interface.

Description

-- ~3 ~ ~

FIELD OF THE INVENTION

This invention relates to improYed electrooptical displays especially of the guest-host type and to liquid crysta71ine admixtures and to chiral additives useful as hos~ media in svch displays for providing improved con- !
~rast at lo~/ applied voltage in the mu1tiplexed mode. .
I . . .

The family of electrooptical display devices kno~m generally as guest-i host devices have high potential util~y for information display purposes i such as digital ~atches or clocks, calculators and other ~ns~ruments. The typlcal guest-host device includes a pair of f1at3 parallel ~ransparent substrates carrying transparent electrode segments on the~r fac~n~ surfaoes ! and a m~xture of ne~atic liquid crystal host compounds and a guest dichroic dye compound sealed be~ween the subs~rates and electrodes. In ~:hi s arrangement, ~he guest dye molecules tend to assume the orientation of the host l~quid crystal molecules rela~ive to the spaced substrates. The con-struction and operation of such guest-host electroop~ical display devices ,1 are ~ell known as shown in the He1meier U.S. Pa~ent 335~7,026 issued Decem-I' ber 29, 1970; the Ush~yama U~S. Patent 4,241,33g issued December 27, 1980;
I the Suzuki etOal- UOS. Patent 4,257,S82 issued March 24, 1981, and the l, Togashi U.S. Patent 49266,8S9 issved May 12, 1987.
!i I
jl In one type of guest-hos~ display, the host liquid crys~al molecules and therefore the guest dye molecules are aligned with ~heir long axis ¦ paralle1 ~homogenous3 ~o ~he spaced subs~ra~es in ~he unac~iva~ed (off) I
I' l ... .. . _ . .. . . ...... , . , .. ,,, .. , . , .. . , . . , . ", , .

31~ ~

¦ s~te. Ho~ver, when an electric fleld is genera~ed across the electrodesegment, the liquid crys~al molecules al~gn perpendicular (homeotropically) ¦ to the substrates as do ~he guest dye molescules. Since the dichroic dye I molecules absorb only light whose electric vector lies along the long dye ¦l axis, the homeotropically aligned dye molecules absorb little light and the ¦, llquid crys~aldye ~i~ture betw~en act~va~ed electrode segmen~s appearsessent~ally colorless or transparent to the vie~Yer of inciden~ light. Of course, homogenously aligned areas of the m~xture appear co70red or dark as a result of the perpend~cular orientation of ~he dye molecules ~o ~he inci-i dent light. A display having light or colorless digits or symbols on adark or color background is thereby provided.
!l .
¦, Hohever~ guest-host display devices of this type suffer from a serious ¦I dra~back in that9 at best, the homogenously aliyned dye molecules w~
absorb only 50~ of the light ~ncident upon the device, ~hereby resulting ~n ¦! poor display contrast This limitation is due to the fact that only one ! polar~zation d~rect~on of ~he incident light has its electric vector Il aligned along the long axis of the dye molecule while ~he other po7ari-¦, zation direction has its electric vector aligned transverse to ~he long dye axis. One attempted solution ~o thi s drawback has been to use ~ kno~m I substrate surface alignment techniques such as rubbing to induce a 90 , th~St (helix) in the long axis of the homogenously aligned liquid crystal ¦- molecules from one substrate to the other ~uch as in the well known twisted I nematic liquid crystal electrooptical display devices, for example, see the ¦ Taylor and Whi~e U,S. Patent 3,833,287 issued September 3, 1974 and Coates and Gray U.S. Patent No. 4J145J114 issued Marc~ ~09 1979. The purpose-of this helical molecular s~ructure js to ensure that no mat~er what the ori entation of the electric vector o~ the incident 1iqht, there wil7 be a dye Il .

3~
I' ~
I' molec~le at some distance between the spaced substrates with ~ts long axis ! parallel to the vector to effect absorpt~on. Thus, absorption of 90g or ,I more of the incident light can be effected. Unfortunately, however, as is i well known in conventional twis~ed nematic l~quid crystal devlces, the host liq~ld crystal exhibits a posltive birefringenoe and ~ends to act as an op- ¦
tical waveguide so that the polark dt~on of l~ght transmitted through the devlce is thisted. 6ues~-host displays made with positive birefringent 1iqu~d crystal compounds are opt k ally equivalent ~o a non-t~ sted homo-~enously aligned guest-host device hn~h ~he attendant poor contrast.

The possibility of utilizin~ a host liquid crystal or mixtures thereof ,, hnth ~inimal birefringent properties in such twisted guesthost display il devices in order to 1ncrease contrast was ~nitially proposed by Taylor in the Journal of Applied Physics 45(11), November 1974 at page 4,721. How-ever, a practical mixture of liquid crystal compounds with low enough bire-frin~ence has not up to the present time been known or developed by prior ' art workers. The cyclohexyl-cyclohexane compounds first synthesized by ! Eidenschink et.al., Ansew. Chem. 133, p. 90 (1978) probably have low enough I b~refringent properties for twis~ed guest-host displays but m~xtures con- j ¦, tain~ng these compounds are generally smectio rather than nematic a~ room jl temperature and also have low dielectrk anisotropy wh~ch results ~n ¦l unacceptable slow response times during display opera~ion and/or higher operating voltage~ The phenyl cyclohexanes disclosed in the idenschink et.al. U.S. Patent 4,130~502 ~ssued December 1978 require higher ~ol~age levels and prDvide lower contrast wh~ch are not satlsfaotory for multi-plexed electrooptical displays.

¦ t~.S. Patent `lumber P, j322,354 in the name of Howard Sorkin and of common asslgnee here~n th d~scloses liquid ll c7~stal compounds o~ the fonnula:

R {~}~CN

~here R is alkyl, ~lltoxy~ aryl, ~ryloxy, carboxy or carboxy es~er. These compounds have a ~ery low electric~1 threshold vol~age of approxlma~ely 0.6 volt, and rela~vely low opt~cal birefr~ngence of ~ n equal to 0.1.
East Gennan Patents 13~2 and 139,867 disclose tl~oxane compounds sf the general fonnula X ~O_~ Y ~nd l~quid c~ystalline admixturei con-tai ni ng sllch compounds.

U.S. Patent 4~298 j528 in the name oP Nkholas Se1;hofer and of common assignee herew~th describes l~quid compounds of the fonnula: `
Rl ~ R2 when R and Rl can be the same or d~fPerent straight cha1n al!~yl or alkoxy group. These compounds ~xh~b~t extremely low opt~cal birefringenee values of ~ n ~qua1 to 0.0~ and ~n some cases D.005, Liquid crystalline com-pounds hav~ng khe formula:

C~} ~ Rl ~here R and Rl are as deser~bed are Ako disclosed ~n the referenced patent as well as U.S. Patent Nos4 4,323,504 and 4,323,473 and are use-ful in raisiny the clearing point of liquid crystallineoa~positions.

~, ~ ., ~

jl~L~L~3 3 ~ ~ I

ilAlso~ u.s. Patent Nu~Tber 4,325,830 .
in the name of Nicholas Sethofer and of common assignee here-th discloses three ring liqu~d crystalline compounds of the fonmula:
I, R ~ ~ ~ ~ R2 ! where Rl is typk ally an alkyl group and R2 ls ~ypically an alkyl,alkoxy, ni~ro or cyano group and r~ng N can be a benzene or cyflohexyl j'l ring. These compounds are also useful In raising the clear~ng polnt of liquid crys~all~ne compositions.

. U.S. Patent 4,200,580 issued April 29, 1979 to Ying Yen Hsu and of com- l ii mon assignee herewith d1soloses compounds of the formula: i ~{7~ ~ 11 where Rl is a straight chain alkyl of 1 ~o 10 carbon ato~s and R2 is klyl, alkoxy, acyloxy, alkyl carbona~o having 1 to 10 carbons~ CN or ~0,~_ .i u.S. Patent 4,313~878 i~ In the name of the same inventor and also o~ con~non assignee herewi ~h , describes 11qu~d crystall~ne compounds of the fonmula:

Rl~ CO--~ R2 ¦~ where Rl and R2 ~re as descr~bed In the U.S. Pat. 4,200,5ao.

~i' .
~11 ~ I

31~ 1 l I
I

I Compounds of the 1,3 dioxane type having phanmaceutical use are dis-closed ~n the Rhodes et.al. U.S. Patent 4,085,222 Issued April 18, 1978.
These compounds, however, do not exhibit 1iquid crystall~ne behavior and are not useful in electrooptic dlsplays.
Il , Chiral or ch~ral containing addi~ives for 7iquid crystalline composi-i tions are also known. For example, the Coates et.al. U.S~ Patent 4,195,916 issued April 1~ 1980 ~llustrates chiral esters and their use in electro-j optic displays. The Gray et.al. U.S. Patent 4,2199256 ~ssued August 26,,~ 1980 discloses compounds of ~he cyanophenyl-alkyl substituted bicyclo i (2c2.2) octane type where the alkyl substituent may con~ain a chiral cen-ter. Three-ring eompounds, ln particular trans-4-alkylcyclohexane-1- car-boxylio acid esters and ester derivatives of l~carboxy4-alkyl substituted bicyclo (2.2.2) octane where the alkyl group may include a chiral center ¦ are taught in ~he Coates et.al. U.S. Patent 4,11~,647 issued September 123 1978 and ~he Gray et.al. U.S. Patent 4,261,652 issued Aprll 14, 1981, respectively.

Co~monly used chiral additives such as those of the cholesteryl non-Il anoate type produce a short helical molecular pitch in l~quid crystalline ,I mixtures but exhibit a weak dielectric anisotropy. Certa~n known optical1y active compounds such as 4-cyano-41- [2-methyl ) butylb;phenyl (CB~
¦ ava~lable from BDH9 Ltd.) produce only a moderate helical pltch and exhibit only modera~e d~electric anisotropy which properties have not been adequate for low voltage, multiplexed operat~on, ~

- , .. -- !
,. I

3~
1~
What ~s s~ill needed is a liquid crystalline host composition having substant~ally lower optical birefringence than ourrently available mixtures ' along with other required properties and improved chiral adcditiYes compat-! ible w~th the host composit~on to provide a guest-host eleotrooptic display hrth improved contrast, e.g. a contrast ratio of 3:i at voltages of about 3 volts.
, l SUMMARY OF THE INYENTION
. . ~

I One object of the lnvention is to provide a host liquid crystalline ¦, composition having low optical b~refring~nce, low electrooptical threshold , voltage, low saturation voltage and stPep saturation cur~e.

I' Another ob~ect of ~he invention is to provide a novel chiral compound ¦l useful a~ an additive in liquid crys~alline composltions, especially those provided herein~

Il Still another object cf the invention is to prov;de a phasechange chol ¦l esteric guest host l~quid crystalline composition incorporating the novel . chiral additive and one or more pleochroic dyes and exhibiting improved contrast such as at least a 3:1 contrast ratio in at least the biplexed ~ode o~ operation ~t voltages of 3 volts or below.

I St~ll another object cf the invention is to provide a guest-host elec-¦ trooptical display device which includes oontrast-enhancing alignment means.
l l . ' : `

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Il .
1~
¦ The host llquid crystalline composit~on of the ~nvention includes the follow~ng compounds wi~h the spec~fied formu7as: ¦

" (I~ ~1 ~ ~ Rl where Rl and Rl are alkyl espe-¦l cially straight chain, more especially Rl is e~hyl and - ¦' is n pehtyl group, . I,' ' i R2 - ~ ~ ~ R~ where R2 and R~ -are alkyl, espe ially straight chain, more espec~11y where R2 is ethyl and Rl ~s n-pentyl group.

i~ (III) R3 - ~ _ ~ CN where R3 is a7kyl, especially ¦I straight chain, more especially n-butylg n-pen~yl~ n-hexyl , and n-heptyl groups.

~ (IV) R4 ~ ~ ~ Rl where R~ and R~
:. ' are alkyl, especially str~ight ehain~ more especially where R4 ~s n-propyl and Rl is either n-pentyl or n heptyl I gr~up~. ~

~V) R~; --C~ - ~ CO-(~CN where R~ is alkyl~
especially straight chain, more especially either of j n-propyl, n-butyl or n penty1 groupsO

I Compounds I-Y are present in the trans ~somer conf~quration.

!.
_ ~ .

~:~L85~31:~ , !l In a particular preferred embodiment of ~he host nematic liquid crys-! tall~ne composition, compound I ls present in an amount of 5 to 25 ~ight i percent; compound II, 5 to 25 ~ight percent; compounds I~I total 40 to 80 , weight percent, compounds IY total 10 to 40 weight percent and compounds Y
¦~ total 5 to 15 ~ght percent.
Il . ...................................................... Il In a particular more preferred embodi~en~ of ~he host liquid crystal-lj line compositions, the compounds are present as follows:
¦ Weight %

! 5-ethyl-2-(4-pentylcyclohexyl)-1,3-dioxane 5-25 1 5-ethyl-2-[4-(pentylcyclohexyl)cyclohexy1]-1,3-dioxane 5-25 , 5-butyl-2-(4-cyanophenyl)-1,3-dloxane 10-30 ! 5-pentyl-2-(4-cyanophenyl)-1~3-dioxane 10-30 Il S-hexyl-2 (4-cyanophenyl)-1,3-dioxane 10-30 ¦, 5-heptyl 2(4-cyanophen~1)-1,3~dioxane 10~30 I ~-propyl-2-r4-(4-pentylcyclohexyl)-phenyl]-1,3-dioxane 5-25 ¦ 5-propyl-2-~4-(4-heptylcyclohexyl) phenyl]-1,3 dioxane 5-25 ¦, 4 cyanophenyl~4'-15~propyl-1,3-dioxan-2-yl) 1-5 !~ 4-cyanophenyl~4'-(5-butyl 1,3-dioxan-1-yl) 1-5 ¦¦ 4-cyanophenyl-4'-(5-pentyl-1,3-dioxan-2-yl) 1-5 ii The novel chiral compound of thP invention has the fonmula:
i *
¦ YI- CH3-CH2-CH-CH2 ~ ~ CN
c~3 . I ln .

~1~9311 ~

where * represents an asymmetr~c carbon atom. This chiral compound is use ' Ful as an additive ~o l~quid crystalline compos;tions, especially hostliqu~d crystalline compositions described above, ln additi~e amounts of about 1 to 10 weight percent.
l !
¦¦ The present invention also provides an fmproved phase-change choles-I teric guest host electroopt~cal co~posiklon as a result of including in the aforement~oned host composit~on the chiral additive descrlbed above along w~h one or more pleochrofc dyes.

The presen~ invention also provides an improved guest-host electro- I
optical display device havi ng contrast enhancing alignment means on the i display substra~e means, preferably ~n combination with the inventive chol-esteric guest-host composition.

BRIEF DESCRIPTION OF THE DRAWINGS
! I '' . :
i''' Il Figure 1 is a graph of light transmission Yersus voltage for ~he exem-¦~ plary nema~ic liquid crystalline composi~ion of the Example I.
11l l Il Figure 2 is a similar graph for ~he cholesteric liqu7d crystalline com-!1 position obtained by adding 3 weight peroent of the no~el chiral additive ¦¦ ~o the composi~ion of the Example I.

¦ Figure 3 is a simi~ar ~raph for the cholesteric guest-host liquid crys-¦ talline composition obtained by adding a.8 weight percent of pleochroic dye i to the composit~on of Fig. 2, ' ,1 1 31~ 1 .
¦I F~gure 4 is a schematic ~llustration of an electrooptlcdl device of the guest-host type embody~ng featunes of the ~nvention.

F~gure 5 is an-enldrged view of Fig. 4 illustrating schematically the preferred tilt angle of the guest-host molecules to the display substrates.

!~ Figure 6 ~s a graph of light transmission versus voltage for the dis-i play of Fig. 5 ~fith the cholesteric guest-host composition of the ~nven~ion.ll l DESCRIPTION OF PREFERRED EMBODIMENTS

ffl The compounds I through VI set forth hereinabove may be prepared as ~ollovs: ;

~H2~H O
R C/ + C ~ R~ R ~ Rll- H20 ,, CH20H H

¦I where R are alkyl ehains, pre~erably straight chain ~ith C2 through C5 !l and the optically act~ve 2-methylbutyl isomer (in compound VI) and OHC -R~ are:

OHG - ~ C$Hll OHC ~ ~ C~H

OHC ~ CN

I
.
i.

OHC ~C~ C~

OHC ~ C7H15 OHC ~CûO~}CN

Ex~ct synthetic prooedures for ~he ~ypes of compounds I ~h~ough V1 can be . ~ound ~n the following uOs. patents:
'.

Compound I - uOS. Patent No. 4,298,528.
Compound II ~ u.s~ Patent Nos. 4,323,504 and `' 4,323,~73.
Co~pound III - U.S. Patent No. 4~322~354.
j, ~ompound IY u.S. Patent No. 4,325~830.
Compound Y - u-S~ Patent No. 4 ,313,878.

~; An ~llustrat~on of the preparation of ~he noYel chiral c~mpound ~s pro-v1ded ~n the follcwlng E~ ple:

! 13 1~ .

931~

EXAMPLE

(-~) 5-(2-methyl) butyl-2(4-cyanophenyl)~1,3-dioxane ~COOC2H5 COOC2H5 A) ~Br ~ CH 2_ 3 ~ R~C ~KBr ~

I II

Reduction ~ H20H J

R-CH + C2H50H

III

where.
R=CH 3 - C~I 2 - CH- CH
c~3 The alkylation of a malonic acid ester was carried out with potassium carbonate in dimethyl formamide, reaction mixture being stirred for one week at room temperature to yield about 90% of compound II. Optically active (2-methyl)-bukyl bromide was purchased ~rom Aero Chemical Co., NewarX, New Jersey. For full descripton of above synthetic step see U.S. Patents 4/298~528 issued November 3, 1981 to N. Sethofer. Reduction of alkyl malonic ester was carried out with lithium aluminum hydride in diethyl ether, by the method described in Fieser & Fieser:
"Reagents fox Organic Syntheses," Vol. 1, p.584.

J

~1893111.

I,/CH2~
B~ R-CH ~ OHC ~ - CN ~ R - ~ ~ CN + H20 ,~H20H ~' III IV Y
where ~' Final step ~n the (~) 5-(2-methwl) butyl-2~4-cyanophenyll-1-3-d;Qxane i synthesis eonsists of condensatisn reaction, where aldehyde and 1.2 molar excess of optically active diol are refluxed ~nth the catalytical amQunt of p~toluenesulfonic acid in benzene or toluene. Reac~ion is completed when ¦, calculated amount of water is col1ected in the attached Dean-Stark trap, e.g., on the average of 40 to 60 minutes. Reaction mixture is then cooled, ~ washed first with 10~ NaOH solution in water, then several times with i ~later" l~yers separated and ~olvent evaporated.
,. .
! Reaction yields mixture of trans and Ci5 ~somers of desired dioxanes, ¦1 ~nd on the a~/era~e, about 5 to lOX of other impurities. The latter can bc ,i easily removed by crystalizat~on from methanol. Separation of isomers ~. (usually in 3:1 ratio for trans-c~s in raw material ) can be accomplished i' either by repeated crystal k ation fro~ hexanes or by employing of chroma-!! tographlc methods~
l!
~
Compound IV, i .e. ~ 4-cyanobenzaldehyde, was purchased from Aldrich Chem~cal Co., catalog No. C8~960-9.

Il . . I

l l 3~ 931~ i I
The host liquid crystalline composition of ~he present invention is illustrated by means of the followfng example which is included for pur-poses of i71ustra~ion rather ~han limitation:

EXAMPLE .
Weight %

C2H5 {~} C5H1 1 10. 0 5-C<;~{~ C5Hll 8~0 C4Hg~ ~ CN 20.0 5 11{~ CN 14.0 ¦ C6H13~o3~CN 17.0 C7H15{~{Or CN ?4.0 C3117-C~}Cs~ll 3~0 C3Hg {~ C7H15 5.0 ~1 C3~7 {~C-0 ~--CN 1.5 ¦I C4Hg~$_c-o~ CN 1.3 ¦, ~ ~ CN 1.2 16.
Il . ' ,1 Il ~

~ 3~
.` I .

Typ~cally, host liqu~d crystalline mixtures for multiplexable guesthost d~splays require the follo~ling character~stics, namely, low op~ical b~re , fringence~ extre~ely low electrooptical threshold and saturation voltage, . broad temperature range and ~ts own high order parameter ~n order to assure I low voltage operation of the f~nal guesthost complex in multlplexed dis ¦ plays. The adm~xture sho~m in Example I exhibits. melt~ng po~nt of abou~
!1 -20C (~ryetall~ne to nematic transs~t~on temperature), clearing point of 1 7250C (nematic to isotropic trans~t~on ~emperature); ~ ~ of ~ 097 (op-¦ t~cal b~refringence) at ~2~C, vfscosity about 30 ep dt 25C. The , electrooptlc character~stics of th~s admixture in a display w~th a 11 to 12 ¦¦ mic~n plate spacing and w~th 30C/12 SiO evaporation angle alignmen~
li layer (i.e. about 1/20 nloleoular sur~ace tilt angle) were as follows:
. 1 . I
! v~o(70~ saturation~ ~ O.9~Y
YgO(9O~ saturation) ~ 1.5Y
response ~me (ON) 5 ~10 mS
response ~ime (OFF) ~ 100 ms i ¦ The above electrooptical data were obtained usillg known tw~ted nematic ¦ type cells, i.e. w~th 90 twist and 2 crossed polarizers. Figure 1 is a ¦ representat~on of the electr.oopt~cal properties of the nematic admixture of ¦¦ Exa:lple I.
j, .
I~ Preparation of ~he comp1exed ~uest-host mixture whkh ls suitable for low Yoltage, mult~plexed operat~on wi~h contrast exceeding 3:1 ra~o requires selection of a chiral ad~it~ve which w~il have ~oth an ex~reme7y ¦ large positive dielectric aniso~ropy and will produce a shor~ ~olecular L8~3 Il hel~cal pl~ch when mixed w~th a nemat~c l~qu~d rrystal~ The aforement~oned 1, commonly used ch~ral add~t~ves, such as. cholesteryl momanoate (CHN) type or ¦1 "opt~cally active compounds" as 4-cyano-- 41-(2-methyl~- butylbi-phenyl(CB-15) have not proved sa~is~actory in meeting all of these charac-terist~cs, as menti~ned already here~nabove.

Therçfore, one of the objects of present invent~on is to proYide an ¦ imprQved chlral addit~on f~r the here~nabove descr~bed mlxture. Appl k ant !I have d~scovered ~hat the compound (+)5-(2-methyl)butyl-2(4 cyanophenyl~-¦l 1,3-dloxane(DPDX) possesses the requ~red character~stics, e.g. d~electric ¦! anisotropy (~ ~ ) is greater than ~17, melting po~nt is 58.8C9 mono-l! tropic ~l-C~)57.SC; ~ H=5.0 K CAL~lOLE, and cholesteric pitch of i approx~mate7y one mfcron when present ln an amount of 7 weight percent in ¦ the above descr~bed nemat~c compos~t~on (Example I).
l .
F~gure 2 ~s a plot of the electrooptk al propert~es of a phasechange sholesteric-nematic liqu~d mixture obtained by introducing 3 ~e~ght percent of ~he abo~e described chiral compound to ~he composi~ion of Example I.
Thi s da~a wPre determ~ned in a tbl sted nemati c type di splay (90 twist) ¦ wlth ~ 11 to 12 m~cron plate spac~ng and w~h 30~12 S10 evaporation ¦¦ angle alignrnent layer, i.e. ~ 1~20 molecular sur~a e t~lt angl . Two ¦. crossed polarlzers were used~ Figure 2 shows that under ~dent1cal align-!~ ment colld~tions the electric21 threshold was raised by 1 volt ~th 3X OPDX
i addition, wh~le rat)o of V5AT an~ma~ns vSrtually unch~nged.

A guest-host electroopt~cal composit~n ~5 prov~ded by further intro-¦ ducîng at least about 0.3 weight percent, preferably about 0.8 to about 1.2 ¦ we~ght percent of a pleochrolc d~ye lnto the phase~change cholesteric ¦ nemat~ c composi t~ on compr~ s~ ng Exampl e I pl us i;he novel chi ral addi ti ve ij I
i.

~8~

(i.e. 3 weight percent). Preferably, a pleochroic dye is used having an order para~eter of greater than 0.7 as determuned by conventional techniques.
A preferred guest~host composition employs about 0.8 weight percent of the dye con~cund:

~ N N - N - ~ / 2~l5 This dye compound is described more fully by Uehida et.al. in Mol. Cryst.
Liq. Cryst., Vol. 34, (Lett.), pp. 150-158 (1977). Of course, those skilled in the art will appreciate that other kncwn pleochroic dyes or mix-tures of dyes can be employed.

Figure 3 is a graph depictLng the electrcoptical properties of the preferred guest-host composition descriked above, i.e. Example I plus chiral additive (3 w/o) and dye (0.8w/o). This data was generated using a display like that used to ge~erate the electrooptical data of Figs. 1 and 2 with ~he exceptian that no polarizers were used or required. Also, as in previous displays, the properties were measured in the transmission nDde. The display exhibited lisht digits on a blue background with a contrast greater than 3 at a voltage of 3 volts. Some contrast increase beyond 3 volts can be attributed to homogeneous surface alignment; i.e. 30 SiO evaporation on OIlR of the display plate surfaces.

Fig. 4 is a schematic view of a basic structure of an electrooptical device of guest-host type according to the in~ention. The device can be driven in ~;rect dri~e or nultiplexed mcde. Particular test device was driven in ~iplexed mcde, respective rms voltages being 0.9V and 2.4 Vh~, frequency 32~z.

l ~ 31~ j . I

The device consists of a cell ~Ath gldss substrate 1 and spacer 3 (in this part~cular case 12 microns),. Transparent electrodes 2 (typical1y ~nd~u~n oxide) and the substrate 1 are covered with surface alfgning SiO
l~yer 4 wh~ch arranges the surP~ce l iqu1d crystal dye molecu1ar layer so ~hat a 10~ to 40 tilt (measured relatlve to the substrate surface) ~s aeh~eved (in this part~cular case t~lt angle was 20 to 23~). Figure 5 is an enlarged v~ew showing ~he tilt angle of ~he molecules relative to ~he subs~rate. -The guest-host compos~ion 5 (in ~his particular case the preferred mixture descr~bed above produces a hel k al structure ~n ~he off sta~e, thus ntens~fy~ng the color of the backgr~und by exposing more qye molecules for light absorpt~on.. The helix also controls ~he electroopt~cal threshold and steepness of the saturation curve (see Fig. 6) thus allowing the dev~ce to be multiplexed. Upon applying the req~ired ele~r~c field between the electrodes 2p the cholesteric structure of layer 5 ~ 5 disruptPd and mole-cules align perpendk ularly wlth their long axes to the substrate e.g.
shown by reference numeral 6 ~n Fig. 4. Above described surface mol~cular pre-ti 7t of 20 to 23 ass1 sted to a great extent ~ n oreati ng more com-plete homeotrop~c alignment under the electric ~ield9 thus diminishing coloration of energized segments and increas7ng the contrast of ~he device~ Commonly used homegenous surface alignment ~or rather lsw tilt angle up to 4) of the prlor art penm~ts residual l~yers of 1iquid crystal and dye molecules to remalin in the parallel direction to the glass substrate even after electrical fièld is appliedJ thus diminlshlng the c~n-trast ratio at low voltages.
'~' . .

3~
I' ll The elec~rooptical device of ~he ~nvention can be employed ~n both transm~ssive and reflect~ve modes. For the latter, Eastman Whit~ RePlec-i ~ance coat~ng ~coi~er~ially ava~lable) was used as d~ffusing reflector.
¦ Other d~ffusing reflecting mater~a7s can be used by those skilled in the !a~.
.

I I , '

Claims (5)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE PROPERTY OR
PRIVILEGE ITS CLAIMED ARE DEFINED AS FOLLOWS:

1. A guest/host nematic liquid crystalline composition having low optical birefringence and low threshold voltage compris-ing:
(a) host nematic compounds: trans-alkyl pentylcyclohexyl dioxanes, trans-alkyl cyanophenyl dioxanes, trans-alkyl pentylcyclohexyl cyclohexyl dioxanes, trans-alkyl alkylcyclohexyl phenyl dioxanes and trans-cyanophenyl alkyl dioxanyl benzoates, (b) a chiral nematic compounds having the formula:
, and (c) a guest pleochroic dye.

2. A guest/host nematic liquid crystalline composition having low optical birefringence and low threshold voltage compris-ing:
(a) host nematic compounds in weight percent:
(b) a chiral nematic compounds having the formula and (c) a guest pleochroic dye.

3. A guest/host nematic liquid crystalline composition having low optical birefringence and low theshold voltage com-prising:
(a) host nematic compounds in weight percent:
(b) a chiral nematic compound having the formula , and (c) a guest pleochroic dye.

4. The composition of Claim 1, in which said trans-alkyl pentylcyclohexyl dioxanes is trans , said trans-alkyl cyanophenyl dioxanes are trans , said trans-alkyl pentylcyclohexyl cyclohexyl dioxanes is trans , said trans-alkyl alkylcyclohexyl phenyl dioxanes are trans and C7H15 and said trans-cyanophenyl alkyl dioxanyl benzoates are

5. The composition of Claims 1, 2 or 3 in which said pleochroic dye has the formula: